I am using mongodb 3.2 (On ubuntu 14.04) and trying to query a large dataset (about 388 millions of little documents). During my aggregation tests I found that mongo sometimes uses 100% in one core and sometimes not and as a consequence, the time of response varies a lot.This CPU usage can also vary from an aggregation query to another (for example : if a want to aggregate more documents).

Please help me to understand better the functioning of this framework.

On CentOS, top shows per process memory using in kb or mb. On Fedora 19, top shows per process memory usage by the byte. Is there an option or setting to make Fedora's top (from procps-ng) work like CentOS' (from procps).